1. Field of the Invention
This invention relates to pressure relief in a high pressure and high temperature microwave digestion assembly. The assembly is particularly adapted for use in a microwave oven and permits safe and effective chemical digestion of sample material for the purpose for chemical analysis.
2. Description of Related Art
Microwave heating and digestion of organic samples in enclosed high pressure and high temperature vessels has greatly shortened the amount of time required to perform chemical analysis. Ventable containers are especially useful for holding analytical samples which are being digested by treatments with strong chemicals, such as nitric, hydrochloric and sulfuric acids. In such heatings and digestions it is often important to keep the digestion vessel sealed to prevent contamination and to prevent losses of materials and reaction products. As heating and digestion proceed the internal pressure of the container normally increases, as material therein is vaporized. Typically, several vessels are used at one time, with a control vessel having temperature and/or pressure sensors therein for providing such information to the operator, which is intended to be representative of temperature and pressure in other sample vessels as well. However, despite the availability of temperature and pressure data, excessive pressure can develop in one or more vessels. To prevent damage to the vessel by development of excessive pressure, a rupture diaphragm or membrane can be used to vent gas and or liquid at a pressure lower than that which will cause significant damage to the container (and possibly to operators using it).
Microwave digestions of various materials, utilizing strong digesting agents, such as acids, are known and open vessel digestions of this type are described in 47 Analytical Chemistry 1475 (A. Abu Samra et al., 1975) and 50 Analytical Chemistry 1021 (P. Barrett et al., 1978). Closed system microwave digestions have also been practiced, as reported in 56 Analytical Chemistry 2233 (Nadkarni, 1984) and in Bureau of Mines Technical Progress Report No. 120 (S. A. Matthews et al., 1983). Savillex Corporation has marketed a lidded vessel for microwave digestions which is made of Teflon.RTM. PFA, obtainable from E.I. DuPont de nemours, Inc. and CEM Corporation has marketed pressure controlling accessories for use in microwave digestions in conjunction with its MDS-81 and MDS-81-D microwave systems.
U.S. Pat. No. 4,672,996 describes a relief valve for use on a container for material being digested in a microwave apparatus. A self-regulating valve positioned under a cap is threaded onto the vessel. The valve includes a pressure deformable, resilient wall member having a fluid vent port and an obstructing member which cooperates with the wall member to open the valve.
Other relief valves are disclosed in U.S. Pat. Nos. 4,400,401; 4,474,211; and 4,493,444.
Another pressure vessel is disclosed in U.S. Pat. No. 4,904,450. A replaceable, rupturable pressure release membrane is provided having predetermined rupture pressure. The membrane covers a vent to seal the container, and will rupture and release pressure within the container if that pressure reaches the predetermined rupture pressure.
Another microwave digestion vessel is disclosed in U.S. Pat. No. 4,613,738. This vessel includes a valve assembly including a ball valve with a vent opening in the lid of the vessel. When internal pressure becomes high enough to overcome the spring coefficient, the ball is unseated off the ball seat, and pressure flows around the circumference of the ball and through a vent.
U.S. Pat. No. 4,882,128 discloses a microwave digestion vessel which includes a sliding seal which is displaced from a sealing position into a non-sealing position when internal pressure exceeds a predetermined value.
U.S. Pat. No. 4,993,529 relates to a microwave digestion vessel including a valve assembly formed by a U-shaped exhaust hole in the cap. Pressure inside the digestion vessel forces a plunger up against the diaphragm, subsequently releasing pressure through an exhaust hole in the cap.
U.S. Pat. No. 5,230,865 discloses a ventable container for digestion including a rupture diaphragm in the venting passageway and enclosure for the ventable container. The diaphragm is tightenable into place so that the container is sealed and may be moved when desired to open and vent the container.
While the relief valves and membranes disclosed in the prior art permit improved control and safer microwave digestion, these devices can limit or restrict the available pressure relief. Digestion of samples containing considerable organic material or minerals that yield gaseous products undergo exothermic reactions that result in large pressure and temperature excursions in closed vessels. Strong mineral acids used in mixtures or in combination with additional oxidizing agents such as hydrogen peroxide increase the possibility that resulting pressures and volumes can exceed pressure relief available.
Depending on the pressure developed in the vessel during digestion, the relief valve or membrane may open or vent, but the vapors and entrained liquid flow through the vent or relief port may exceed the discharge capacity. Thus, the valves and venting mechanisms in the prior art have a definite risk that vapor or liquid will not be adequately vented until the vessel ruptures or fragments explosively.
Another limitation of existing microwave digestion vessels having rupturable membranes is that the membranes typically have been somewhat isolated from the sample in the reaction chamber. If the membrane was not at the same temperature and pressure as the sample, there was less predictability and more uncertainty as to rupture pressure.
Another limitation of microwave digestion vessels resulted from residual pressure after the digestion has been completed. Even if the membrane does not rupture, there still may be residual pressure of several hundred p.s.i in the vessel after digestion. This residual pressure can make it difficult to open the vessel and even dangerous when manually opening the vessel. For example, a wrench or other mechanical device may be required to unscrew the cap, even after the vessel is cooled down before it is opened.